This application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 2005-047402 filed in Japan on Feb. 23, 2005, the entire contents of which are hereby incorporated by reference.
The present invention relates to a high frequency unit that includes a shielding case for shielding a wiring substrate on which a high frequency semiconductor electronic component is mounted.
There have been proposed communication formats for receiving signals in the same frequency band via different communications means. For example, in terrestrial TV broadcasting that uses RF signals as ground waves, a terrestrial CATV (cable antenna TV) receiving format that receives ground waves via CATV and a terrestrial direct receiving format that directly receives ground waves are known. As electronic devices for selecting one of these two formats by switching between them, devices using an RF signal selection switch as a high frequency unit are being used.
FIG. 7(A) and FIG. 7(B) are diagrams schematically illustrating a conventional high frequency unit, with FIG. 7(A) showing its top view, and FIG. 7(B) showing a cross-sectional view taken along the line indicated by the arrows X-X in FIG. 7(A). FIG. 8 is a bottom view of the high frequency unit shown in FIG. 7(A) and FIG. 7(B).
The high frequency unit shown in FIG. 7(A), FIG. 7(B) and FIG. 8 may be, for example, an RF signal selection switch, which includes, as its external terminals, a terrestrial input terminal 101 as a high frequency input terminal, a CATV input terminal 102 as a high frequency input terminal, an RF output terminal 103 as a high frequency output terminal, a selection signal terminal Tsw into which selection signals Ssw are input, a power supply terminal Tcc to which a power supply voltage Vcc is supplied, and a ground potential terminal GND held at ground potential Ve. These external terminals are attached and fixed to a shielding case 104 made of metal (e.g., copper). The shielding case 104 surrounds the peripheral edge of a wiring substrate 105 to provide shielding (electromagnetic wave shielding).
A high frequency relay 115, which is a large mechanical component serving as an RF signal selection circuit, is mounted on the front face of the wiring substrate 105. The selection signal terminal Tsw, the power supply terminal Tcc and the ground potential terminal GND are connected to a wiring pattern (not shown) that is formed on the wiring substrate 105.
A core wire 101L of the terrestrial input terminal 101 and a core wire 102L of the CATV input terminal 102 are connected to the high frequency relay 115, mounted on the front face of the wiring substrate 105, on the rear face of the wiring substrate 105. Further, RF signals that have been selected by the high frequency relay 115 are output to the RF output terminal 103 via a broadband amplifier 116 (see FIG. 9) and a core wire 103L.
The wiring substrate 105 can be divided into a terrestrial input block BL-A as a block corresponding to the terrestrial input terminal 101, a CATV input block BL-B as a block corresponding to the CATV input terminal 102, and an RF output block BL-C as a block corresponding to the RF output terminal 103.
The shielding case 104 contains shield separation plates 104w and 104p for separating the terrestrial input block BL-A, the CATV input block BL-B and the RF output block BL-C from one another. On the rear face of the wiring substrate 105, a shield separation plate 104q for separating the terrestrial input block BL-A and the CATV input block BL-B is attached afterwards as a discrete component. Further, leg portions 104b of the shield separation plate 104w are formed protruding to the rear face of the wiring substrate 105. To the shielding case 104, a shielding lid 104ra is attached on the front face side of the wiring substrate 105, and a shielding lid 104rb is attached on the rear face side.
FIG. 9 is a block diagram showing a circuit block of the high frequency unit shown in FIG. 7(A), FIG. 7(B) and FIG. 8. Components identical to those shown in FIG. 7(A), FIG. 7(B) and FIG. 8 are given identical reference numerals, and their description has been omitted where appropriate.
The circuit block shown in FIG. 9 includes, as its main components, a high frequency relay 115 and a broadband amplifier 116 that are mounted on the wiring substrate 105. The high frequency relay 115 selects RF signals from the terrestrial input terminal 101 and the CATV input terminal 102. The broadband amplifier 116 amplifies the selected RF signals, and outputs them from the RF output terminal 103. The high frequency relay 115 is connected, for example, to a selection signal terminal Tsw into which selection signals Ssw are input, and a power supply terminal Tcc to which a power supply voltage Vcc is supplied. The broadband amplifier 116 is connected, for example, to the power supply terminal Tcc.
When the selection signal Ssw input into the selection signal terminal Tsw is a signal selecting the terrestrial direct receiving format, the high frequency relay 115 is connected to a terminal 120. That is, the RF signals from the terrestrial input terminal 101 will be output from the RF output terminal 103. On the other hand, when the selection signal Ssw input into the selection signal terminal Tsw is a signal selecting the terrestrial CATV receiving format, the high frequency relay 115 is connected to a terminal 121. That is, the RF signals from the CATV input terminal 102 will be output from the RF output terminal 103.
As an electronic unit including an electromagnetic wave shielding plate, an electronic unit is known in which an electromagnetic wave shielding plate is attached onto an electronic circuit board (see e.g., JP2002-9478A).
Since the terrestrial CATV receiving format that receives ground waves via CATV and the terrestrial direct receiving format that directly receives ground waves use RF signals in the same frequency band, the RF signal selection switch is required to have very high isolation properties so that the signals will not cause interference.
However, although the high frequency relay as a single component can achieve favorable isolation, it is expensive and has a large size, which has been the cause of preventing its cost reduction and miniaturization. Further, due to its large size, it has been difficult to completely separate input terminals of the high frequency relay on the front face of the wiring substrate, using a shielding plate.
Additionally, although it is necessary to use a rigid isolation structure also on the rear face of the wiring substrate to prevent isolation degradation resulting from radiation from the lead (core wire) portion of the frequency relay and the RF input/output terminals, it has been difficult to mount a complex shielding plate having excellent shielding properties due to cost and workability reasons. This has led to reduction in product quality and yields.
The present invention was made in view of such circumstances, and it is an object of the present invention to provide a highly reliable, low-cost high frequency unit that can provide reliable shielding, has a simple structure, is easy to manufacture and can achieve miniaturization and high performance.